Project Summary Human hematopoiesis is a complex, dynamic process that requires intricate regulation of multiple gene pathways involved in lineage commitments, leading to development of multiple mature blood cell types. Multiple studies in mice and humans report age-related defects in early hematopoiesis and functions defects in the mature aged immune cell populations, which leads to impaired immune responses to pathogens. These defects are especially important because population studies estimate that nearly 40% of European and US populations will be over 60 years of age by 2050. Previous studies in our lab indicate that loss of expression of the DNA-binding protein ARID3a leads to hematopoietic developmental skewing away from lymphoid lineage progenitors, indicating that ARID3a is important for normal hematopoiesis. Preliminary data from aged human peripheral blood shows decreased frequency of ARID3a-expressing hematopoietic stem cells (HSCs), leading to decreased numbers of multipotent progenitors and multi-lymphoid progenitors, but no alterations in numbers of myeloid progenitor cells when compared to healthy young peripheral blood. The proposed studies seek to expanded the preliminary phenotypical data of ex vivo young and aged human peripheral blood hematopoietic progenitor cells for statistical significance, evaluate and compare developmental potential of young ARID3a- expressing, young ARID3a-deficient and aged HSCs in vitro and in vivo. We will further examine the gene expression differences between young and old, as well as between ARID3a expressing and ARID3a deficient HSCs by RNA-seq. Differentially expressed genes will be further analyzed for ARID3a binding and chromatin remodeling by ChIP-seq and ATAC-seq analyses. These sequencing techniques will be the focus of the training portion of this proposal. Finally, we will evaluate the capacity of young ARID3a-expressing, young ARID3a-deficient, and old HSC engrafted humanized mice to mount immune responses to immunization with PC-KLH and survival following inoculation with S. pneumoniae. The overall goal of the proposed studies is to determine the effect of ARID3a expression in HSCs on immunosenescence and innate immune responses. Completion of these studies will stratify HSCs based on ARID3a expression, identify ARID3a-regulated genes important for aging, and identify potential therapeutic targets for improving immunological function in elderly individuals.